The hippocampal mossy fiber system has been proposed by many to be an ideal system for analyses of the function of opioid peptides. This anatomically well defined (and electrophysiologically accessible) axonal system has been found, by the applicant and others, to contain enkephalin (ENK)-, dynorphin (DYN)- and (in guinea pig and mouse) cholecystokinin (CCK)-like immunoreactivity. It is not known whether these peptides are colocalized or are present in distinct mossy fiber populations. There is evidence that mossy fiber ENK and, less certainly, DYN and CCK content is altered by seizure activity and a range of studies indirectly suggest that these peptides are involved in the regulation of epileptiform activity as well. It is the goal of the proposed research to further refine our understanding of the fine localization of ENK-, DYN-, and CCK-like immunoreactivities in the mossy fiber system through the application of immunoelectron microscopic techniques and to evaluate the recipricol interaction of these mossy fiber peptides with epileptiform activity. Specifically the latter studies propose to: 1) quantify the influence of seizure activity on DYN, ENK, and CCK immunoreactivities in hippocampus by radioimmunoassay; 2) evaluate the impact of the seizure-induced increase in mossy fiber terminal-contained ENK (and possibly DYN) immunoreactivity on opiate receptor binding patterns; and 3) use the in vitro hippocampal slice preparation and a penicillin-induced interictal-spike paradigm to evaluate the (positive or negative) epileptogenic effects of mossy fiber-contained peptides. The anatomical (EM and autoradiographic) studies should help clarify unresolved issues as to the 'place' of ENK, DYN, and CCK in hippocampal circuitry and thereby limit the range of considerations as to the local physiological role of these substances. It is hoped that data from the proposed studies will combine to advance our understanding of the involvement of hippocampal peptides in limbic seizure activity and, more generally, the interaction between physiological activity, terminal-contained peptide concentration, and peptide receptors throughout the CNS.